1. Field of the Invention
The present invention relates to a dust-free and pore-free, high-purity granulated polysilicon, to its production and its use.
2. The Prior Art
Granulated polysilicon is an alternative to the polysilicon produced using the Siemens process. The polysilicon produced in the Siemens process is in the form of a cylindrical rod of silicon which has to be broken up to form what is known as chip poly. If it is appropriately purified again, this is a time-consuming and expensive process. Prior to further processing, granulated polysilicon has bulk material properties and can be used directly as raw material, e.g. for the production of single crystals for the photovoltaics and electronics industry.
Granulated polysilicon is produced in a fluidized bed reactor. It is produced by fluidizing silicon particles by means of a gas flow in a fluidized bed, the latter being heated to high temperatures by means of a heating apparatus. The addition of a silicon-containing reaction gas causes a pyrolysis reaction to take place at the hot particle surface. Elemental silicon is deposited on the silicon particles, and the diameter of the individual particles grows. The regular extraction of grown particles and addition of smaller silicon particles as seed particles allows the process to be operated continuously, with all the associated benefits. Silicon-containing starting gases described include silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4) and mixtures of these gases with hydrogen. Deposition processes of this type and corresponding apparatuses are known, for example, from WO 96/41036, DE 3638931 C2 (corresponding to U.S. Pat. No. 4,786,477) or DE 199 48 395 A1.
The granulated silicon obtained from the deposition process is distinguished by a high purity, i.e. a low level of dopants (in particular boron and phosphorus), carbon and metals.
U.S. Pat. No. 4,883,687 has disclosed granulated silicon which is defined on the basis of the grain size distribution, the boron, phosphorus and carbon contents, the surface dust content, its density and bulk density.
U.S. Pat. No. 4,851,297 has described a doped granulated polysilicon, and U.S. Pat. No. 5,242,671 has described a granulated polysilicon with a reduced hydrogen content.
U.S. Pat. No. 5,077,028 describes a process in which a granulated polysilicon which is distinguished by a low chlorine content is deposited from a chlorosilane.
The granulated polysilicon which is currently produced on a large scale has a porous structure, resulting in two serious disadvantageous properties:                Gas is included in the pores. This gas is released during melting and disrupts the further processing of the granulated polysilicon. It is therefore attempted to reduce the gas content of the granulated polysilicon. However, as described in U.S. Pat. No. 5,242,671, this requires an additional working step, which increases production costs and, moreover, results in additional contamination of the granulated polysilicon.        The granulated polysilicon is not particularly resistant to abrasion. This means that fine silicon dust is formed during handling of the granulated polysilicon, e.g. when it is being transported to the user. This dust is disruptive in a number of ways:        it interferes with the further processing of the granulated polysilicon, since it floats when the granulated polysilicon is being melted;        it interferes with transportation of the granulated polysilicon within the production installation, since it causes deposits to form on pipelines and leads to blockages in valves and fittings;        it is a potential contamination carrier on account of its large specific surface area.        
The abrasion leads to losses even during production of the granulated polysilicon in the fluidized bed.
The production based on monosilane as silicon-containing starting gas, is currently customary. In addition to the abrasion in the deposition process, this disadvantageously also results in the direct formation of dust as a result of a homogeneous gas phase reaction followed by recrystallization.
Although some of this ultrafine dust can be separated from the product, this also entails additional work, loss of material and therefore increased costs.